The resonant frequency for a quartz resonator is determined by the thickness of the quartz between two electrodes of the resonator. U.S. Pat. No. 7,237,315 to Kubena et al. for a Method for Fabricating a Resonator, which is incorporated herein by reference and commonly assigned with the present application, describes a method for fabricating a quartz resonator. In that prior art, plasma dry etching technology is used to form the resonator structure with soft photoresist used for masking. However, there is a substantial difference in the quartz thickness required for quartz resonators with greater than 100 MHz resonant frequencies (several microns) and the quartz thickness required for quartz resonators at lower frequencies at or below the VHF frequency band (several tens or hundreds of microns). Low frequency quartz resonators have much greater quartz thicknesses and so the fabrication methods used for high frequency quartz resonators are not always appropriate for low frequency quartz resonators.
FIGS. 1a to 1l show a prior art method of fabricating quartz resonators. Although the method fabricates many resonators at once, FIG. 1 shows only one resonator being fabricated. The starting materials are a single-crystal quartz wafer 20, a silicon handle wafer 30, and a host substrate 40 as shown in FIG. 1a. The process begins by defining and etching a cavity 32 in a silicon handle wafer 30, as shown in FIG. 1b. Then, a top-side electrode 22 and tuning pad metal (Al or Au) 25 and 26 are deposited onto the single-crystal quartz wafer 20 in FIG. 1c. Next, the two wafers are brought together using a direct bonding process using low temperature bonding/annealing in FIG. 1d. A series of processes including wafer grinding/lapping, chemical-mechanical-planarization (CMP), plasma etching and chemical polishing are used to thin the quartz wafer 20 down to a thickness of typically less than 10 microns, for a desired resonant frequency, in FIG. 1e. The 10 microns or less thickness is appropriate for quartz resonators with resonant frequencies above 100 MHz. Next, photolithography is used to pattern via holes 28 and 29 in the quartz wafer 20 and holes are etched through the quartz wafer to stop on top-side metal 25 and 26 of aluminum (Al) or gold (Au) and then metallized to form through-wafer conductive vias 28 and 29 in FIG. 1f. A bottom-side electrode 24 and bottom-side metal 34 and 35 are then metallized in FIG. 1g. Then the quartz wafer 20 is patterned and etched to form a resonator in FIG. 1h. Protrusions 41 are etched into the host substrate 40 in FIG. 1i, and metallization patterns, including bonding pads 42 and 44, are defined on the host substrate in FIG. 1j. The quartz/silicon pair produced in FIG. 1h is then bonded to the host substrate 40 using either a gold to gold (Au—Au) or gold to indium (Au—In) compression bonding scheme in FIG. 1k. Then the silicon handle wafer 30 is removed with a combination of dry and wet etches, resulting in the quartz resonators being attached only to the host wafer 40, as shown in FIG. 1l. 
This prior art method uses spin coating of a soft mask (photoresist) for patterning metal, quartz and silicon structures. For example, photolithography is used to pattern via holes 28 and 29 in the quartz wafer 20 and holes 28 and 29 are etched through the quartz wafer to stop on top-side metal 25 and 26 of aluminum (Al) or gold (Au), as shown in FIG. 1f. However, for low frequency resonators that require thick quartz substrates, the aluminum or gold metal layer is no longer an adequate etch stop layer due to the long plasma etching required to form via holes in a thick quartz substrate.
Commercially available low frequency quartz resonators are fabricated as separate discrete components due to the conventional processes employed to make them. Fabrication as separate discrete components increase their cost.
Some commercially available low frequency quartz resonators are fabricated using wet etching. Wet etching of quartz is notoriously slow and only allows circular or rectangular quartz blanks to be fabricated due to the asymmetrical etching profiles that result from preferential crystallographic etching rates. For example, a Z-axis etch rate is at least 500 times faster than those of x- and y-axis etch rates. Also, if the crystals are rotated to form various cuts for temperature compensation, then there are further limits on the shapes that can be formed using wet etching. Dry etching allows arbitrary shaped resonators to be formed and provides 3-4× improvement in etch throughput. However, in either case the methods of the prior art to fabricate low frequency quartz resonators are not amenable to wafer or chip scale integration of quartz resonators with other electronic circuits to form, for example, oscillator circuits. This raises the cost of using prior art low frequency quartz resonators.
What is needed is a method of making low frequency quartz resonators that is amenable to wafer production to thereby lower cost and allow chip scale integration of quartz resonators with other electronic circuits. The embodiments of the present disclosure answer these and other needs.